1. Field of the Invention
The present invention relates in general to a ring trip detector for a subscriber telephone line, by which an on-hook state or an off-hook state of the telephone line is detected. More particularly, the present invention relates to an improved ring trip detector which avoids false alarms and quickly detects the off-hook state, regardless of variations in subscriber equipment or loop lengths.
2. Description of the Related Art
To alert telephone subscribers to a telephone call, telephone sets are equipped with a calling device (a bell or other acoustic transducer) which is responsive to an alternating current electric calling signal applied to the subscriber's line at the telephone exchange. At the same time the called party receives the calling signal, the calling party receives tones indicating that the call has been switched through.
When the called party answers the telephone set, it is necessary for the calling signal to be removed from the called party's line, preferably near instantaneously, to avoid an intense sound being emitted from the telephone transducer. Also, the tones indicating a call must be removed from the calling party's line to indicate that the called party has answered.
Removal of the calling signal and tones indicating a call from the lines is known as "ring trip," and ring trip detection determines when removal should take place. Recently, stringent requirements have been placed on ring trip detectors. Ring trip detectors must detect an off-hook state (when the telephone set is answered) within a short period of time, for example 200 ms. The ring trip detector must also be immune to other line conditions, such as off-hooks and other power spikes, which occur for less than 12 ms. Ring trip detectors must consistently meet these conditions regardless of whether the "loop" connecting the subscriber's telephone is short and has a low resistance or is long and has a high resistance. The loop resistance may be as great as 2,000.OMEGA.. Further, ring trip detectors must pass a test to ensure that the ring trip detector will avoid erroneous ring trips, in which an 8 .mu.F capacitor and a 10 k.OMEGA. resistor are substituted in parallel for the subscriber's telephone set. In this test, a ring trip must not be detected.
In telephone switching systems, line cards serve as the interface between outside lines and switching equipment. The line cards may be provided at the central office or at intermediate equipment, such as a digital loop carrier, which conveys the line traffic to the central office. The line cards contain ring trip detectors thereon. A large number of lines may be interfaced at a single location. For example, there may be 20,000 line interfaces and hence 20,000 ring trip detectors at a single location. Because of the great number of ring trip detectors and line cards necessary in switching equipment, the line cards and ring trip detectors are very cost sensitive pieces of equipment. Even a few cents difference in the cost of switch card components must be weighed against the resulting overall great increase to the total cost.